- it costs money and attention
- good is not the same as perfect (there are some casualties from time to time)
They protect against being "policed" by big world countries.
Eg. if Ukraine still had nuclear weapons, Russia would not have been invading them (or are they "protecting" them, as promised when they took their nuclear arsenal for destruction?). If Iran or Iraq had nuclear weapons, they would not have been bombed by US.
Israeli nukes are the main reason we haven't had regime change in Tel Aviv at the hands of a Turkish/Egyptian/Saudi/Iranian coalition. Israeli nukes are why Iran has had to settle into a pattern of slow, distant, annoyance via proxy forces (which lack a capability for existentially challenging the IDF).